Suppressor grid tube



Aug. 9, 1938. H. J. McCARTHY 2,126,492

SUPPRESSOR GRID TUBE Filed March 26, 1935 CURRENT PLATE CURRENT lNVENTOR ATTORNEY PLATE VOLTAGE Patented Aug. 9, 1938 UNITE STATES PATENT OFFICE SUPPRESSOR GRID' TUBE Application March 26, 1935, Serial No. 13,047

Claims.

This invention relates to electron discharge tubes, and more particularly to tubes having a suppressor-grid, such for example as pentode tubes or the like.

- An object of the invention is to provide an improved form of electron-discharge tube having an electron emitting cathode and a plurality of grids, one of which is capable of being main tained substantially at cathode potential without being connected to the cathode.

Another object is to provide an electron-discharge tube with a suppressor-grid which is treated to maintain the potential thereof at a chosen value without connecting the grid to any other electrodes of the tube.

A further object is to provide an electron-discharge tube having a cathode, anode. control grid, shield-grid, and a third grid which is treated so that when the tube is connected in circuit it exhibits the usual pentode characteristics without the necessity of connecting the said third grid to the cathode.

A feature of the invention relates to a pentode type tube having a floating suppressor-grid which is especially treated to avoid the necessity of connecting said grid to the cathode or to any other electrode of the tube.

A still further feature relates to the novel organization, arrangement and character of parts which go to make up an improved pentode type tube.

Other features and advantages of the invention not specifically enumerated will be apparent after a consideration of the following detailed description and the appended claims.

In the drawing, Fig. 1 is a diagram of characteristic curves to explain the action of a suppressor-grid according to the invention.

Fig. 2 is a schematic diagram of a tube in circuit embodying features of the invention.

Fig. 3 is a typical plate voltage-plate current characteristic curve of the usual pentode with its suppressor-grid returned to the cathode.

While the invention will be schematically disclosed herein as embodied in one specific type of circuit arrangement, it will be understood that this is done merely to explain the inventive concept and to illustrate one mode of using a tube embodying said concept.

Referring more particularly to Fig. 1 the curve A represents the relation between potential and current flow to or from a metallic grid positioned in an electron stream from a cathode. In this figure the ordinates represent magnitude of the :current flowing into or out of the grid while the abscissae represent the potential of the grid with respect to a chosen base potential for example the potential of the cathode which is represented in Fig. 1 as zero. From an inspection of Fig. 1, it will be seen that the grid current is zero not only when the potential of the said grid is near zero but also at the points X and Y where the grid has a positive potential with respect to the cathode. It has been found that if the said grid is left floating in the electron stream, that is if it is not conductively connected to the cathode or to any of the other elements of the tube, it may assume any one of the three potentials at which the current to the grid would normally be zero if it were conductively connected to the cathode and its potential varied. Thus it might assume a potential near that of the cathode, a potential X or a potential Y. It is the customary practice however, to connect the suppressor grid of a pentode type tube to the cathode or at least to maintain it by a suitable conductive circuit arrangement at or near cathode potential.

CurvesB and C of Fig. 1 represent the grid voltage-grid current relations of a grid treated or processed in accordance with the present in vention. From these curves it will be seen that the grid current will be zero only at one potential which is approximately the same potential as the cathode. At any other potential there will be a positive grid current. In other words when the floating grid is processed in accordance with the present invention it can only assume one potential at which the current is zero, namely near zero or cathode potential. I have found that it is possible to employ such a floating grid in an electrondischarge tube by treating the surface of this grid in a suitable manner, examples of which will be disclosed herein.

One of the most useful applications of such a grid is in connection with the so-called pentode tube. As is well known, this type of tube employs a suppressor-grid between the anode and shieldgrid to suppress or reduce to a negligible value the secondary emission from the plate which normally tends to reach the shield-grid. However, it will be understood that the invention is not limited particularly to a pentode type tube but is capable of embodiment in any tube where a grid is interposed in an electron stream and is subjected to a considerable electron bombardment resulting ordinarily in an appreciable secondary emission therefrom. Thus for example in the pentode tube where the suppressor-grid is usually interposed between the shield-grid and the anode, the electrons from the cathode reach the suppressor-grid at a relatively high velocity. Likewise the electrons that pass the suppressor-grid strike the anode with suflicient velocity to release secondary electrons therefrom. The primary function of the suppressor-grid therefore is to prevent these secondary electrons reaching the shield-grid and thus affecting the potential thereof. However, the electrons that actually strike the suppressor-grid also release secondary electrons which emission of secondary electrons preable coating to control the secondary emission therefrom, it is possible to maintain this grid at or near cathode potential notwithstanding that the said grid is being struck by relatively high velocity electrons, and is free or floating.

Thus there is schematically shown in Fig. 2 a highly evacuated vessel I, containing an electronemitting cathode 2, a control-grid 3, a shield-grid 4, a suppressor-grid 5 according to the invention, and an anode 6. While the cathode 2 is shown as of the indirectly heated type adapted to be heated by a separate heater filament I connected to a source of heating current (not shown), it will be understood that a filamentary type cathode may be employed. For a detailed description of the structure of a tube of this general type, reference may be had to Patent No. 1,950,456. Connected across the control-grid 3 and the cathode 2 is any suitable form of signal inputcircuit represented by the coupling coil 8,

the grid 3 being preferably negatively biassed with respect to cathode 2 in any well known manner for example by the automatic biassing resistor 9 shunted by the condenser ill. The anode 6 is connected through a suitable output coupling unit II to the positive pole l2 of a suitable source of relatively high steady potential. Likewise the shield-grid 4 is connected to the positive terminal I2 of the high voltage supply. "The suppressor-grid 5 which is positioned between the shield-grid 4 and the anode 6 is left floating, that is to say, it is not connected to any other element of the tube or circuit. In accordance with the present invention the suppressor-grid 5 has its surface provided with a coating of a material or ,materials to control the secondary emission the cathode 2 even though the said grid is entirely disconnected from said cathode. Probably one reason. for this action is that when the grid 5 is bombarded by the electrons from cathode 2 ordinarily there may be an excess of secondary electrons emitted from the said grid 5 which results in a variation of the potential of this grid rendering it positive with respect to the cathode 2. I have found that by a suitable character or thickness of coating on the grid 5, it is possible to maintain this potential substantially constant or at least substantially the same as the potential of cathode 2. An explanation for this phenomena is that since the treated grid always receives more electrons than it loses, if it be positive with respect to the cathode, it will continue to receive electrons until its potential is such that it receives no more electrons. This potential is near that of the cathode.

Other methods of treating the grid 5 to achieve the above results may be employed for example the grid might be coated slightly with ceramic or any other material that would reduce the secondary emission. By the expression pentode tube as employed herein is meant a tube having an electron emitting cathode, an anode, a control grid, a shield grid and a suppressor grid located between the shield grid and the anode, and wherein the plate current is determined primarily by the control grid and shield grid potentials and the suppressor grid is employed to protect the shield grid from secondary emission from the plate.

What I claim is:

1. Electron discharge apparatus comprising an evacuated tube containing an electron emitting cathode and an anode between which a stream of electrons passes, a grid situated in said stream at a point where it is subjected to bombardment by high-velocity electrons so that normally it would emit an appreciable number of secondary electrons, said grid having its surface treated to reduce secondary emission therefrom, circuit connections for applying a potential between said cathode and anode whereby said grid is bombarded by said high-Velocity electrons, an output circuit connected to said anode and cathode, said grid being entirely insulated from both the oathode and the output circuit and assuming substantially cathode potential while insulated therefrom.

2. Electron discharge apparatus comprising an evacuated tube containing an electron emitting cathode, an anode, a control grid adjacent the cathode and connected thereto, another grid adjacent the anode but insulated therefrom said other grid having its surface treated to reduce secondary emission therefrom, circuit connections for applying a potential across said cathode and anode whereby said other grid is bombarded by high-velocity electrons, said other grid being entirely insulated from both the cathode and output circuits so that by reason of its treated surface and said bombardment it assumes substantially cathode potential while insulated therefrom.

3. Electron discharge apparatus comprising an evacuated tube containing an electron emitting cathode, an anode, a control grid adjacent the cathode, a shield grid, and a suppressor grid adjacent the anode, means to impress a potential across the cathode and anode whereby said suppressor grid is bombarded by high-velocity electrons, said suppressor grid having a coated surface to reduce secondary emission therefrom whereby said suppressor grid assumes a potential near the cathode potential while insulated from both the cathode and anode.

4. An electron discharge tube of the pentode type having asuppressor-grid which is conductively disconnected interiorly and exteriorly of the tube and which is treated to reduce secondary emission therefrom to the point Where said grid assumes a potential near cathode potential by reason of said coating when subjected to highvelocity electrons, and while conductively disconnected from the remaining electrodes of the tube.

5. Electron discharge apparatus comprising an evacuated tube of the pentode type having input and output circuits, wherein the plate current is determined primarily by the control grid and shield grid potentials, the suppressor grid having a coating to reduce secondary emission therefrom whereby said grid performs its usual suppressing action while disconnected from both the input and output circuits.

' HENRY J. MCCARTHY. 

